1. Field of the Invention
The present invention relates to a power conversion device that drives a motor and more particularly relates to a motor control device with a vector control function provided with the function of compensating for a deceleration condition produced by abrupt change of load.
2. Description of the Related Art
A typical layout of a motor control device of the voltage inverter source is shown in FIG. 1. The motor control device shown in FIG. 1 is of the vector control type having speed feedback and current feedback control whereby an input speed reference signal xcfx89r* is subjected to feedback calculation with a speed feedback signal xcfx89xe2x80x2 calculated by a speed sensor 1, primary flux angle calculator 2 and differentiator 3, and the result of the calculation is converted to a torque reference signal Tr* by the speed regulator (ASPR=Automatic Speed Regulator) 4, and the q axis torque current reference signal Iq* is calculated by dividing this value by the secondary flux reference "PHgr"2*. Also, the d axis current reference signal Id* is calculated by a field weakening regulator (AFWR=Automatic Field Weakening Regulator) 5 and a flux saturation pattern setter 6, from the speed feedback signal xcfx89xe2x80x2. The respective final current reference signals Id, Iq are generated by feedback calculation in respect of these current reference signals Id* and Iq* with the d axis and q axis current feedback (F. B.) signals Idxe2x80x2, Iqxe2x80x2, and are output as respective voltage references Vd*, Vq* by respective current regulators (ACR=Automatic Current Regulator) 7, 8 of the d axis and q axis. Based on these voltage references Vd*, Vq*, power converter 10 then converts the DC voltage Vdc that is supplied from the DC power source to the desired AC voltage Vac by means of element gate firing (ignition) pulse instructions from 2-3 axis co-ordinate/PWM converter 9 and outputs this, thereby driving motor (IM=Induction Motor) 11 by supplying the desired current thereto.
In this construction, in the d axis field current Id, which is the field component of the motor in vector control, the secondary flux reference "PHgr"2* is calculated in accordance with the speed feedback signal xcfx89xe2x80x2, in accordance with a single field pattern in the field weakening regulator 5. This field pattern is set as a fixed value determined for each individual motors connected to the current control device. When a speed feedback signal xcfx89xe2x80x2 of a certain magnitude is input, the d axis field current Id of the motor is thereby calculated in accordance with the field pattern. Also, the slippage angle xcex8s is calculated based on the primary flux angle xcex8r calculated by the primary flux angle calculator 2 and the q axis torque current Iq that supplies the torque required by the motor and the secondary flux angle xcex8o required by the motor is found by adding the primary flux angle xcex8r, which is the flux angle of the actual motor. Motor 11 can thus be driven by supplying any desired voltage to motor 11 by using these to effect conversion from the d-q axis components to the three-phase AC output components.
In a conventional motor control device as described above, even if the speed reference xcfx89r* is fixed, when there is abrupt variation (change) of the load such as for example on biting (threading) into rolling mill material, as shown in FIG. 2, a drop occurs in the motor speed (called xe2x80x9cimpact dropxe2x80x9d); Accordingly, a q axis torque current Iq was added such as to convert the motor speed which had thus dropped to the target speed prior to impact drop while concurrently motor accelerating torque was applied to motor 11 by calculating the secondary flux angle xcex8o taking into account slip angle (slippage angle, xcex8s and the respective voltage references Vd* and Vq*, by means of d axis and q axis current regulators 7 and 8. In this process, there was a lag on the rise of the torque, due to restrictions an the output range of the output voltage of the control device (converter). Thus, in a conventional motor control device, a considerable time was required to recover the drop in motor speed produced by occurrence of impact drop.
Such a drop in motor speed produced by impact drop is disadvantageous both from the point of view of product quality maintenance and economically in rolling plants and there was a strong demand for improvement of motors in respect of such impact drop.
Accordingly, one object of the present invention is to provide a novel motor control device with a vector control function capable of recovering rapidly drop in motor speed produced by impact drop, by improving the rise of motor current when impact drop occurs produced by a steep change in load.
The foregoing objective is achieved by a motor control device with a vector control function having the following construction.
Specifically, according to the invention, a motor control device with a vector control function that inputs a speed reference signal from an external plant control device and that outputs a desired AC voltage and frequency in accordance with this speed reference signal comprises: load condition evaluation means (unit) that identifies the load condition of the motor from load information input from said external plant control device; d axis field current limiting means (unit) that limits the d axis field current Id to a certain limiting amount when the load condition evaluation means (unit) identifies that the load change is steep; and d axis field torque compensation means (unit) that compensates insufficiency of the motor torque produced by suppression of the d axis field current Id by said d axis field current limiting means (unit) by increasing the q axis torque current Iq.
With a motor control device with vector control function according to the invention, when the amount L of actual load change that is picked up from an external plant control device exceeds a reference level L* of load change amount per prescribed unit time (per dozens of mm seconds) that is pre-set to serve as an impact drop detection level, the d axis field current Id is suppressed to a prescribed limiting amount. The d axis voltage reference Vd* and hence the motor and armature voltage Eac of the motor are thereby suppressed exclusively during a prescribed field limiting period Td. A large potential difference (voltage margin) is thereby generated between the converter output voltage Vac and armature voltage Eac of AC motor, thereby speeding up the rise of output current of the motor. Subsequent recovery of the motor speed after impact drop can be speeded up by returning the voltage level to the original d axis voltage reference Vd*.